The present invention relates to a thread wound golf ball. More particularly, it relates to a thread wound golf ball having long flight distance, while maintaining the characteristics peculiar to the conventional thread wound golf ball, i.e. good shot feel.
Many golf balls are commercially selling, but they are typically classified into solid golf balls such as two-piece golf ball, three-piece golf ball and the like, and thread wound golf balls. The solid golf ball consists of a solid core of molded rubber material and a cover of thermoplastic resin (e.g. ionomer resin) covering on the solid core. The thread wound golf ball consists of a solid or liquid center, a thread wound layer formed on the center and a cover of ionomer resin or balata etc. having a thickness of 1 to 2 mm covering on the thread wound layer.
The thread wound golf ball, when compared with the solid golf ball such as two-piece golf ball, has better shot feel at the time of hitting and better controllability at approach shot. The thread wound golf ball is generally approved of or employed by high level golfers, especially professional golfers, who regard the characteristics as most important. However, the thread wound golf ball is inferior in flight distance to the solid golf ball, because the thread wound golf ball has large spin amount and small launch angle.
In the thread wound golf balls, there are two types, such as one comprising a solid center formed from integrally molded rubber material and the other comprising a liquid center composed of a hollow rubber sphere and liquid encapsulated in the sphere. Among them, a thread wound golf ball comprising a liquid center and a balata (trans-polyisoprene) cover is particularly approved of or employed by high level golfers, especially professional golfers, because of good shot feel and excellent controllability at approach shot. The thread wound golf ball comprising a solid center, when compared with the thread wound golf ball comprising a liquid center, has excellent flight distance, because the rebound characteristics of the center also have an effect on the rebound characteristics of the golf ball itself. However, the thread wound golf ball is generally inferior in flight distance to the solid golf ball as described above, and it is required to improve the flight distance, while maintaining the characteristics peculiar to the thread wound golf ball.
In order to solve the problem, a thread wound golf ball comprising a solid center, which has further excellent shot feel and controllability and long flight distance mainly by adjusting the hardness distribution of the solid center to suitable range, has been proposed in Japanese Patent Kokai Publication No. 271537/1997, Japanese Patent No. 2715885 and the like.
However, the thread wound golf ball, which has sufficient flight distance as long as solid golf balls, while maintaining the advantage of good shot feel and excellent controllability peculiar to the thread wound golf balls, has not been obtained. Therefore the thread wound golf ball having further excellent shot feel and controllability and long flight distance is required.
A main object of the present invention is to provide a thread wound golf ball, of which flight distance is improved, while maintaining soft and good shot feel.
According to the present invention, the object described above has been accomplished by employing a thread wound core which comprises a solid center, and adjusting a diameter, surface hardness (JIS-C hardness) and hardness distribution of the solid center, and a flexural modulus and Shore D hardness of the cover to a specified range, thereby providing a thread wound golf ball, of which the flight distance is improved, while maintaining soft and good shot feel.
The present invention provides a thread wound golf ball comprising a solid center, a thread rubber layer formed on the solid center, and a cover covering the thread rubber layer, wherein
the solid center has a diameter of 30 to 38 mm and a surface hardness in JIS-C hardness of 65 to 90, the surface hardness is higher than a center hardness in JIS-C hardness of the solid center by not less than 12,
the cover is formed from a base resin mainly comprising ionomer resin, and has a flexural modulus of 200 to 600 MPa and a Shore D hardness of 60 to 80.
In order to put the present invention into a more suitable practical application, it is desired that the ionomer resin used for the cover have an acid content of 12 to 30% by weight, and 5 to 80% of carboxyl groups in the ionomer resin are neutralized with metal ion.